1. Field of the Invention
The invention relates to an apparatus for determining the physical parameters of figures on a surface, for example the size and position of apertures in a printed circuit board.
2. Description of Prior Art
There are numerous instances in which the physical parameters of figures on a surface must be determined. For example, nearly every manufacturing process includes at one point or another an inspection of the product being manufactured. If the manufacturing process is highly automated, it is also desirable to automate the inspection step. Although the automation of an inspection process may be very difficult or costly to implement, particular types of products are quite amenable to automated inspection.
There have been several machines developed to automatically or semi-automatically inspect printed circuit boards (PCBs) to determine whether they have been manufactured to acceptable tolerances. The manufacturing of a printed circuit board requires the forming of hundreds or even thousands of precisely sized and positioned holes. Each hole must be formed and located to allow for the insertion of an electronic component lead into it or to allow an interconnection to be made to another layer of the PCB by placing electrically conductive material in the hole. If even one of the PCB holes is misaligned or missized, automatic insertion of the electronic components at a later stage of assembly may be disrupted or an improperly formed interconnection may render the PCB inoperative.
One apparatus for inspecting printed circuit boards requires an operator to manually insert a small probe sequentially into each hole in the PCB. When the probe is inserted in a hole, the position of the hole relative to a predetermined reference point is automatically determined by the apparatus. This apparatus has several disadvantages. It requires an operator to manually control the movement of the probe from one hole to the next. In addition, the insertion of the probe into a hole may damage the hole or the PCB. For instance, removal of the probe from the hole may lift a copper foil overlay which may surround the hole from the board. Also, such a machine does not permit measurement of the size of the hole.
Another apparatus for inspecting holes in a PCB uses collimated light beams which are projected through the hole from below the PCB. Optical detectors determine from the light exiting the hole whether the hole is within acceptable positon and/or size tolerances.
Although this apparatus has the advantage of not contacting the board during the inspection process, it does have certain disadvantages. A PCB may be wavy, resulting in holes which are not exactly perpendicular to the wavy surface of the board. This board waviness may be within acceptable tolerances, a permissible deviation, yet a collimated light beam apparatus may indicate that a hole is outside the acceptable range due to the non-perpendicular nature of the hole with respect to the surface. Another disadvantage of such an apparatus is that it cannot be used to measure the parameters of a blind hole, i.e., a hole which does not pass through the board. Another disadvantage to this apparatus is that it may be used only to inspect holes. It would be desirable to also be able to inspect other features or figures on the surface of the board, i.e., terminal pads and copper foil conductors.
Many apparatus which determine the values for parameters of a figure encounter difficulties when the figure is formed by a hole in the surface. For instance, the location of a circular hole may be mathematically referenced with respect to the geometric center of the circle which the hole forms on the surface. However, to determine the center of a circular hole in a surface is complicated by the removal of the surface material at the hole's center. Once the hole is formed, its center can be determined only by inspecting its perimeter. Determining the hole's center from its perimeter, although mathematically straightforward, has been a difficult task for many prior art apparatus.
It is an object of the invention to provide an apparatus suitable for automatically inspecting a nominally flat surface, such as a PCB, to determine if figures on the surface have parameters, such as size, position and shape, within the acceptable margin of manufacturing tolerances. It is a further object of the invention to provide an apparatus for measuring the parameters of a hole in a PCB, even if a blind hole, without causing damage to the board by the inspection process. Another object is to compensate for the nonplanarity of a surface when figures on its surface are being inspected. An additional object is to provide data gathered on the parameters of figures on a surface to an external device for additional processing such as statistical analysis. Other objects will appear to those skilled in the art from the following description of a preferred embodiment.